1. Field of the Invention
This invention relates to electronic circuits for controlling the power to brushless direct current motors, and more particularly to minimizing current spikes in the stator windings of a brushless direct current motor when the stator coils are switched.
2. Description of the Relevant Art
Although the present invention pertains to polyphase dc motors, in general it finds particular application in conjunction with three phase dc motors, particularly of the brushless, sensorless type which are used for rotating data media, such as found in computer related applications, including hard disk drives, CD ROM drives, floppy disks, and the like. In computer applications, three phase brushless, sensorless dc motors are becoming more popular, due to their reliability, low weight, and accuracy.
Motors of this type can typically be thought of as having a stator with three coils connected in a "Y" configuration, although actually, a larger number of stator coils are usually employed with multiple motor poles. Typically, in such applications, eight pole motors are used having twelve stator windings and four N-S magnetic sets on the rotor, resulting in four electrical cycles per revolution of the rotor. In operation, the coils are energized in a sequences such that a current path is established through two coils of the "Y" with the third coil left floating. The sequences are arranged so that as the current paths are changed, or commutated, one of the coils of the current path is switched to float, and the previously floating coil is switched into the current path. Moreover, the sequence is defined such that when the floating coil is switched into the current path, current will flow in the same direction in the coil which was included in the prior current path. Therefore, six commutation sequences are defined for each electrical cycle in a three phase motor. The method and apparatus for operating a polyphase motor direct current motor is more fully explained in U.S. Pat. No. 5,221,881 and is fully incorporated into this specification by reference.
During the phase commutations of a motor, current ripple has been a problem which results in undesirable acoustical noise produced by the motor and unnecessary wear on the motor. Therefore, an important measurement of the performance of a dc motors is ripple which is the rotational acceleration due to non-constant torque on the motor and is a function of current. The relationship between ripple and current through the stator winding is more fully developed in U.S. Pat. No. 5,191,269 and is fully incorporated into this specification by reference.
Another important measurement of the performance of dc motors is the electrical current demand on the host system and host power supply. It is advantageous to system designers to have dc motors which require low average current demands as well as low dynamic demands. A skilled system designer can lower the total power requirements of the system or increase the system performance with an efficient (low average current demand) dc motor. Conversely, an inefficient dc motor can require a system designer to increase the power supply size or give up other system performance features.
Similarly, the dynamic loading of a dc motor on a power supply needs to be considered by the system designer. Excessive dynamic loading can cause a power supply to "crow bar," which is how a power supply protects itself from what it thinks is a short circuit in the system. Additionally, large dynamic loads add noise to the system. The bigger the dynamic load, the more difficult it is for a system designer to protect the system from the temporary voltage spikes associated with the dynamic load. Therefore, it is advantageous for system designers to have dc motors with low dynamic current requirements.
The current flow in a stator winding is typically controlled by the circuit in FIG. 1. It represents the output stage of the current driver for a brushless dc motor. The problem encountered in this configuration is that when the output stage is turned off by moving switch 30 into the "0" position, the feedback loop 60 is open and the compensation capacitor 20 is thus charged to the output level of the error amplifier 10, even if input Vin is brought to 0 volts, due to the intrinsic offset voltage of the error amplifier. Consequently, when the output stage is turned back on by switching 30 to the "1" position, the current on the output stage is only limited by the stator coil 45 and sense resistor 55 until the loop enters the linear mode of operation. Depending on the value of the compensation capacitor 20, the time required to discharge the capacitor may be significant and therefore the duration of the output current spike may also be significant. This, in turn, creates an excessive current demand on the power supply of the system.
In addition to the unwanted load on the system's power supply, the current spike increases the electrical stress to components by causing them to dissipate additional power. Since these devices are often used in laptop or notebook computers which have limited air ventilation for cooling, any additional power dissipation can result in an increase in operating temperature. It is well known in the art that the reliability of semiconductors is inversely proportional to operating temperature which means that any increase in power dissipation in the power stage of the current drivers for the stator windings can result in reduced reliability.
It is an object of the this invention to decrease the high current spike when enabling current delivery to the stator windings of brushless dc motors.
It is further an object of the invention to decrease the dynamic load to the system's power supply caused by enabling current to the stator coils in a brushless dc circuit.
It is further an object of this invention to decrease the electrical stress to the output stage of the current drivers for a brushless dc motor by minimizing current spikes in the stator windings.